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^ Video of the noise. "The deceleration noise" is best heard on video with the windows up.

Recently, i've done a clutch/flywheel job, and noticed that after the install, i've gotten a bad rattle noise that seems to be much worse then the original "subaru deceleration noise" that has been discussed to death.

This is on a lightly modded 04' WRX with ~69k. After a few years, i've had the noisy in-gear deceleration as thousands of other subaru owners do. I recently changed the stock clutch with an OEM style Exedy replacement kit, and a 10lb Cusco flywheel. Installed a new throwout bearing, pilot bearing, clutch disc, and pressure plate. Everything was torqued to spec, cleaned, and lubed according to the manual.

Now, I get a distinct, very loud rattling noise that occurs during in-gear off-throttle coast situations, somewhere above 1800rpm. It kind of sounds like a bolt is loose.

I do remember having a little problem with separating the engine and transmission bellhousing though, as I was a lazy bastard and somehow managed to move the engine forward instead of dropping the transmission. Did I flex the input shaft too much while trying to pry the engine from the transmission? I know a lightweight flywheel will cause more NVH but the noise is quite loud and obnoxious; it is very hard to ignore.

The best description I can come up with, is as if something rotating is rattling or contacting where it shouldn't.

I have the act streetlite flywheel and oem clutch and it is pretty loud off throttle. My wife actually gave the the "OMG WTF is that noise" the first time she heard it. It is normal. It sounds like all sorts of hell is going on.

I don't think you could have been rougher separating the tranny from the motor than I was. Slide hammer/5 pound maul/3 foot prybar.

Are you sure? Haha, I mean, you're right about "all sorts of hell going on" but is it really this bad? If it's normal, then I think I can live with it, but it's kinda funny, my car sounds broke as I decelerate.

Anyhow, I dunno if you flexed the input shaft as much as I did. I think at one point in time the input shaft was not centered in the bearing retainer ("candlestick"?), there was so much pressure on it that it was closer to one side. As soon as the engine was seperated though, the input shaft seemed centered, and unharmed.. But I hope I didn't harm an internal bearing.

I think I will make a video tomorrow and see if I can post it up, just to make sure the sound is normal.

A video would be cool. I would venture to say that it is normal. Mine sounds like a harsh grinding/popping/humming. Your flywheel is 4 pounds lighter than the one I have. I would guess that it would be even louder than my car because there is less mass to dampen any vibration.

Yah, welcome to a lightened flywheel.
Check your wheel pressures, put them at the recommended 36f/32R, it will help a little.
It rattles because with a lightened flywheel you have a lot less inertia to keep everything meshed in the tranny. It seems like some of it is helped by replacing the various bushings which generally tightening up drivetrain.
The tranny/engine mounts, Rear Diff bushings, the tranny crossmember bushings, all seem to help some. Not to mention your entire drivetrain will feel better.

I have this same problem with a Gruppe-S lightened flywheel and ACT street-strip clutch. I've searched around about this, but found nothing stating whether this is harmful over time to anything. I thought harmonics can cause damage over periods of time. Any input anyone?

Well, there's your problem right there! Not really a problem, but the newer USDM wrx gearboxes often sound like hell with a lightweight flywheel... and from what I've read it seems to get worse as the flywheel gets lighter. In fact, subaru was sometimes installing even heavier flywheels under warranty shortly after the US debut of the wrx to calm the decel noise and shudder folks where complaining about (My car's original owner told me that mine got it done a couple of years back). I've heard that noise in another wrx with a 12lb. flywheel before, sounds about right.

Sorry to go off topic, but gotta say, I love your exhaust! What is it?

Hehe.. I hate to bump an old thread up since there are others similar to this and i've since found some resolution, but the exhaust is a HKS Carbon-Ti but it has no resonator and the internal "tuning tube" has been removed.

Well, there's your problem right there! Not really a problem, but the newer USDM wrx gearboxes often sound like hell with a lightweight flywheel... and from what I've read it seems to get worse as the flywheel gets lighter. In fact, subaru was sometimes installing even heavier flywheels under warranty shortly after the US debut of the wrx to calm the decel noise and shudder folks where complaining about (My car's original owner told me that mine got it done a couple of years back). I've heard that noise in another wrx with a 12lb. flywheel before, sounds about right.

Thank You for the reply. I've since read up on this and found relief that it's no big thing to worry about

Are there any lightened flywheels on the market that do not cause this horrendous noise on a bugeye model transmission? I have the Gruppe-S lightened flywheel with the Driveshaft Shop aluminum 1 piece driveshaft and the sound is horrible as well as the vibrations. When I get up to 100+ it feels like the whole car is going to shake apart.

As far as noise goes... nothing really out of the ordinary on mine. I have what I believe to be an STi 12.5lb. flywheel (at least that's what it looks like)... HOWEVER, I also have an STi vIII 5mt from 1996 with the older style case and 4.44FD, so it may have been built somewhat differently (I'm sure their manufacturing methods have changed at least somewhat between 1996 and 2002). I run valvolene 75w90 and 1qt. of pennzoil synchromesh. I did notice a lot more gear noise with Uncle Scotty's cocktail before switching back to valvolene/synchromesh, so I'm not sure if a different fluid may ease the noise a little bit.

They keep "refining" the 5mt... but based on my experience, I'd say they're going backwards. The older trans seems to be less noisey, less clunky on the shifting, and generally more well behaved than any of the newer suby 5mts that I've driven. I think it's just the way gear lash is set on the newer transmissions, plus maybe other manufacturing tolerances involved that cause the decel noise problem.